In recent years, by the development of remote sensing technologies, observational instruments mounted in aircraft and artificial satellites have grown in performance, and the amount of information transmitted from the air to the ground is increasing. In order to efficiently transmit, to the ground, data to be generated in observation instruments having more improved performance in the future, a free space optics (FSO) system that uses an optical frequency band, by which a wider bandwidth can be expected more considerably than by microwaves, has been studied.
In the free space optics (FSO) system, a highly sensitive receiver is required in order to achieve an ultra-long distance transmission from an artificial satellite to the ground. In order to achieve a large-capacity free space optics (FSO) system, it is necessary to employ a high-speed technology for a transmission rate and a wavelength multiplexing technology. In this case, it is efficient to use a common technology with an optical fiber communication technology, that is, to apply an optical transmitting and receiving technology using a single mode fiber (SMF). The reason is that it is possible to use a direct optical amplification technology with low noise and high gain, a highly sensitive digital coherent receiving technology, a high bit rate transmitting and receiving technology, a dense wavelength division multiplexing (DWDM) technology and the like, for example.
An example of a free space optical communication device using such an optical fiber communication technology is described in Patent Literature 1.
In the free space optics (FSO) technology, it is general to make a modulated laser beam with a narrow beam to propagate through the air. On the receiving side, light beams are collected by an optical antenna and propagate through a short-range fiber, and then signal reception is performed. For the purpose of a large capacity, in Patent Literature 1, an optical transmitting and receiving technology using a transmitter including a coherent light source such as semiconductor laser and a single-mode fiber (SMF) is employed.
In a free space optics (FSO) receiver, a wave-front distortion of laser light due to signal propagation through the air near the ground becomes a problem as described below. A beam spot is formed on a focal plane in a collecting unit of the free space optics (FSO) receiver, and a speckle pattern arises on the beam spot due to an atmospheric disturbance. By the occurrence of the speckle pattern, the beam spot diffuses or moves (scintillation) against an ideal focal plane.
In the free space optics (FSO) receiver, optical coupling with a single mode fiber (SMF) is required as a bit rate of a signal increases. However, the above-described phenomenon of beam spot variation becomes a serious problem because it brings deterioration of the coupling efficiency (fade). The reason is that even a slight fluctuation of a beam spot causes a large fade to arise in the single mode fiber (SMF) having a small core diameter; consequently, the loss of received data arises. Therefore, in order to achieve a high capacity free space optics (FSO) system, it is necessary to suppress the fade due to scintillation with increase in a transmission rate.
In order to prevent the deterioration of the coupling efficiency (fade) with the above-described single mode fiber (SMF), the FSO receiver described in Patent Literature 1 is configured to use a single fiber tapered from a large core to a small core or a fiber bundle. Specifically, the FSO receiver described in Patent Literature 1 includes a telescopic collection system, a wavelength demultiplexer, photodetectors, analog-to-digital converters, and a digital signal processor. The FSO receiver has a configuration in which the light is collected from the demultiplexer into a plurality of individual fiber end faces and a tapered fiber bundle or a tapered single fiber concentrates the light into a single output fiber for input to the photodetector.
By this means, a relatively large optical aperture is provided for collecting the optical signal. Thus, there are known a tapered fiber that gradually becomes thinner and employs an adiabatic taper to couple efficiently the collected light into a single-mode output fiber. It is said that one benefit of the large aperture that is afforded by the above-described technique is greater tolerance to beam wander which tends to degrade the performance of the communication system.
Patent Literature 1 discloses a multi-mode fiber bundle obtained by fusing single-mode fibers together. An FSO receiver is disclosed that is configured to collect light on a large aperture surface of the fiber bundle using a collective lens.